The present invention relates to offset path generating apparatus, and particularly to such apparatus useful for generating the tool center path of numerical control (N/C) machines.
In my copending U.S. Pat. application Ser. No. 382,862 filed July 26, 1973 (now U.S. Pat. No. 3,875,382), there is described a technique for generating a two-lobed epitrochoid contour for producing the Wankle rotary engine housing. Among the systems described in that application is an offset control for generating an offset path of motion deviating from a programmed path of motion by a prescribed magnitude. The system in that patent application is particularly described for generating the center path of the tool tracing an epitrochoid contour, but may be used with respect to any programmed path of motion, such as a linear, circular, parabolic, or other mathematically-defined loci. The present application is directed to improvements in the offset path generating system described in that patent application.
Numerical control (N/C) apparatus is generally supplied with program instructions that define a particular profile or contour path with respect to a particular tool-size radius. The program path of the tool center is generally computed off-line prior to use. Accordingly, if a larger or a smaller sized tool is desired to be used than was assumed in determining the programmed path, such as in the case when sharpening a tool or dressing a grinding wheel, it is generally necessary to compute a new program off-line before the N/C machine can generate the path with the changed tool size. Further, if several passes are to be made over the same contour, even though each path is identical except for a change in the size of the cutter assumed in calculating the path, each path has to be repeated in the program thus necessitating a substantial extension in the length of the program and in the memory storage capacity required. In addition, the complexity of the procedure in calculating the tool offset off-line makes it difficult for a machine operator to program an N/C machine directly from a part print.
Further, in the prior art currently in use for N/C systems, tool offsets are generally effectuated by open-loop biasing of coordinate axes with analog voltages. This introduces errors because of the inherent limitations of analog methods. One such limitation is the range of values that can be realized without significant errors. Another, is the error introduced by variations in the ambient temperatures. A further source of error is that when turning on or off the tool offset, or when accommodating path discontinuities or any rapid changes of direction, the axes offset rate of response is dependent on fixed time constants. In addition, analog systems require calibrations performed by skilled personnel.
My U.S. Pat. application Ser. No. 382,862 (now U.S. Pat. No. 3,875,382) describes a digital apparatus for generating drive axes-pulses for producing an offset path of motion offset from a programmed path of motion by a prescribed magnitude. That system, briefly, includes: data input means for inputting data specifying the programmed path of motion and the prescribed offset magnitude; a program path generator generating program axes-pulses defining the programmed path of motion specified by the input data; an offset generator generating offset axes-pulses defining an offset vector of a magnitude equal to the prescribed offset magnitude; and an output mixer algebraically combining the program axes-pulses and the offset axes-pulses and outputting same as the drive axes-pulses for producing the offset path of motion.
Such an offset path generating system is particularly useful for generating the tool center path of N/C machines. Being completely digital, it has no limitation as to the amount of offset, nor does it require any calibrations, nor does its rate of response limit the velocity at which contouring may be performed. It may be embodied in a hardwired control that can be appended to existing N/C systems. Also, it utilizes simple control algorithms to generate Cartesian coordinate axes motion command pulses that are algebraically combined with the axes motion command pulses of the original programmed path and which are of the same pulse weight.